1. Field of the Invention
The invention pertains to the field of variable camshaft timing (VCT) systems. More particularly, the invention pertains to dither frequency control.
2. Description of Related Art
For a variable cam timing (VCT) system, an electronic solenoid is used to drive the spool valve which in turn controls the oil flow which powers the VCT unit. The solenoid is preferably either pulse width modulated or current controlled. A dither signal is imposed upon an input signal to the solenoid for reducing the effects of static and dynamic friction. Usually the dither signal is a small percentage of the overall signal amplitude and a fixed frequency.
However, a relatively high xe2x80x9cfrequency beatingxe2x80x9d problem occurs when the dither frequency of the solenoids will match or in the proximity of the frequency of another part of the system. For example, the frequency of a cam torque signal produced by a valve train of an internal combustion engine may match the dither frequency thereby causing frequency beating. Frequency beating occurs when a first frequency having similar characteristics with a second frequency thereby causing undesirable effects.
It is desirable to reduce the above frequency beating problem and at the same time maintaining a suitable dither signal.
In a VCT system, a change of dither frequency at a neighborhood of frequency beating point specific to a particular engine type is provided.
Accordingly, a method that uses a dither signal for reducing hysteresis effect in a variable cam timing system is provided. The method includes the steps of: a) providing a dither signal having at least two switchable frequencies; b) determining the frequency characteristics of an engine at different speeds; c) determining at least one frequency beating point in relation to a neighborhood of an engine speed; and d) changing the dither signal frequency when the engine is operating within the neighborhood of the engine speed. Thereby frequency beating effect is reduced.